1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to starting a Brushless Direct Current (BLDC) motor, and more particularly, to a device and method for starting a BLDC motor, in which a startup current is maintained at low level when the BLDC motor starts up, thereby reducing startup noise and vibration and startup failure rate.
2. Description of the Related Art
Generally, a sensorless BLDC motor includes a stator on which a coil is wound and a rotor that is provided in the stator and rotates to drive a load. The position of a rotor is detected based on a back electromotive force (EMF) appearing on coil terminals, and a voltage applied to the stator is controlled based on the detected position. Since the sensorless BLDC motor sets the time when the current is to flow based on the back EMF, the motor does not require a Hall sensor for magnetic polarity detection, achieving high reliability.
The sensorless BLDC motor is driven by estimating the position of the rotor based on the back EMF and generating a drive signal based on the estimated position. However, the position of the rotor cannot be detected effectively if the motor runs at low speed. That is, since the position of the rotor cannot be detected when the BLDC motor is in sensorless startup mode, a drive signal must be generated in a manner different from conventional driving methods.
In order to overcome this problem, as shown in FIG. 1, the BLDC motor starts up after an initial Pulse Width Modulation (PWM) duty is determined assuming an initial synchronous speed for starting the BLDC motor (S10 and S20). The BLDC motor continues to start up until a synchronous time set according to the PWM duty passes (S30).
If the synchronous time has passed, it is determined whether the synchronous speed is greater than or equal to a predetermined threshold speed (about 600 rpm) suitable for switching to the sensorless mode (S40). If the synchronous speed is less than the predetermined speed, phase commutation is performed, and the PWM duty and the synchronous speed are increased to drive and accelerate the motor (S50 and S60). If a synchronous time set according to the increased PWM duty has passed, it is determined whether the synchronous speed is greater than or equal to the threshold speed (S30 and S40).
As shown in FIGS. 2a and 2b, the BLDC motor starts up by increasing the PWM duty and the synchronous speed while performing phase commutation until the synchronous speed exceeds the threshold speed suitable for switching to the sensorless mode. If the synchronous speed is greater than or equal to the threshold speed, the running mode of the BLDC motor switches to the sensorless mode (S70). Then, it is determined whether the position of the rotor is being detected (S80). If the position of the rotor is being detected, it indicates that the startup has succeeded, so that the motor continues to run in the sensorless mode (S90). If the position of the rotor is not detected, it is determined that the startup has failed, so that the BLDC motor takes action against the failure (S100).
It is very important for the sensorless BLDC motor to switch from the startup mode to the sensorless mode to achieve stable control. It is necessary to perform the switching to the sensorless mode at the moment when the rotor position detection is normally performed.
However, in the conventional BLDC motor starting method, since the PWM duty is constantly increased even though rotor position detection is not performed normally, the rotor position and the phase commutation time may become desynchronized depending on changes in the load torque of the BLDC motor, and the startup current may become excessively high, thereby causing high startup noise and vibration.
In addition, since the conventional BLDC motor starting method increases the rotor speed as described above, the rotor position and the phase commutation time may be desynchronized, increasing the startup failure rate, if the conventional BLDC motor starting method is applied to a system such as a compressor, whose load torque increases as the rotor speed increases. Also, an excessive startup current may occur, making it difficult to optimize specifications of the inverter.